FIG. 19 illustrates an example of a prior art tracking servo mechanism using the DPD (Differential Phase Detection) system. This tracking servo mechanism is often adopted for optical discs, such as DVDs (Digital Versatile Discs or Digital Video Discs).
A laser beam LB, focused on a signal recording surface of an optical disc 102, is reflected from the signal recording surface. The reflected beam is detected and photoelectrically converted by optical pickup 100 to produce a radio frequency (RF) electric signal, whose waveform corresponds to the embossed pattern of the bit sequence. Here, photoelectric conversion part 104 of optical pickup 100 has, for example, four light-receiving regions A, B, C, D made up of photodiodes. Although not shown in the figure, the light-receiving regions A, B, C, D are divided and arranged so that they fit together obliquely from the four sides. RF signals A, C obtained from light-receiving regions A, C positioned on a diagonal have nearly the same phase as do RF signals B, D obtained from light-receiving regions B, D positioned on the other diagonal. The variation in the phase difference between RF signals A, C and RF signals B, D corresponds to the tracking error. Also, although photoelectric conversion parts 104 are actually within optical pickup 100, for the sake of clarity, photoelectric conversion parts 104 in FIG. 19 are shown individually extracted from optical pickup 100.
The tracking servo circuit comprises analog circuits including analog front end part 106, the first stage, and digital circuits comprising digital front end part 108, the second stage. The two front end parts 106, 108 are usually formed on different semiconductor chips.
In analog front end part 106, gain control amplifiers (GCA) 110, 112, 114, 116 adjust the amplitudes of RF signals A, B, C, D from photoelectric conversion part 104 of optical pickup 100, respectively. Equalizers (EQ) 118, 120, 122, 124 perform wave-shaping and noise reduction functions by emphasizing the high frequencies of RF signals A, B, C, D. Offset cancellation circuits 126, 128, 130, 132 eliminate offsets so that the central levels (zero levels) of RF signals A, B, C, D are aligned. Zero crossing detectors (ZRC) 134, 136, 138, 140 are made up of comparators comprised of op amps. As shown in FIG. 20, the times at which the voltage levels of RF signals A, B, C, D cross the reference level are detected, and a binary signal is output that has rising edges and falling edges corresponding to the zero-crossing times.
Binary signal (A) from comparator 134 (corresponding to RF signal A) and binary signal (B) from comparator 136 (corresponding to RF signal B) are input to phase difference detector 142. And, and, as shown in FIG. 21, a pulse signal with a variable pulse width that corresponds to the phase difference between the two binary signals (A) and (B) is output as a first phase difference signal φAB. This first phase difference signal φAB is output with a positive polarity when binary signal (A) leads binary signal (B), and is output with a negative polarity when binary signal (A) lags binary signal (B).
Binary signal (C) from comparator 138 (corresponding to RF signal C) and binary signal (D) from comparator 140 (corresponding to RF signal D) are input to phase difference detector 144. And, a pulse signal with a variable pulse width that corresponds to the phase difference between the two binary signals (C) and (D) is output as a second phase difference signal φCD. This second phase difference signal φCD is output with a positive polarity when binary signal (C) leads binary signal (D), and is output with a negative polarity when binary signal (C) lags binary signal (D).
First and second phase difference signals φAB and φCD output from the two phase difference detectors 142, 144 are added by an adder 146 which is made up of op amps. Basically, the summation signal φ AB/CD output from adder 146 can be regarded as the tracking error signal of the DPD system. Usually, in order to retrieve the necessary error component for the tracking servo, output signal φ AB/CD of adder 146 is averaged as it passes through low-pass filter (LPF) 148 (FIG. 21), and then passes through gain control amplifier (GCA) 150 for gain adjustment to obtain the tracking error signal sent to digital front end part 108.
Digital front end part 108 is comprised of analog-digital (A/D) converter 152 that converts the analog tracking error signal from analog front end part 106 to a digital tracking error signal, and servo processor (such as DSP: Digital Signal Processor) 154 that outputs a control signal for controlling the position of optical pickup 100 in the radial direction of optical disc 102 to actuator 156 as the feeding mechanism in response to the tracking error signal.
When the beam spot of laser beam LB deviates from the center of the track in the radial direction on the signal recording surface of optical disc 102, the phase difference between RF signals A, B and the phase difference between RF signals C, D will vary corresponding to the tracking deviation magnitude. The phase difference information, that is, the tracking error information, that is contained in output signals φAB, φCD of two phase difference detectors 142, 144, in output signal φ AB/CD of adder 146, and, finally, in the output signal of low-pass filter (LPF) 148, is fed back to servo DSP 154. Corresponding to the fed back tracking error information, servo DSP 154 controls the position of optical pickup 100 in the radial direction through feed mechanism 156, so that the tracking servo circuit locks the beam spot of laser beam LB to a given track. Also, a digital-to-analog (D/A) converter (not shown in the figure) is placed between servo DSP 154 and feed mechanism 156.
As explained above, in the prior art, the tracking servo circuit is comprised of two chips 106, 108, where the size of equalizers (EQ) 118-124 and low-pass filter (LPF) 148 is particularly large. Consequently, the scale of the circuit of analog front end part 106 is large, which is undesirable. Thus, there is a demand that the circuit scale be reduced by integrating the entire tracking servo circuit onto a single chip by digitizing analog front end part 106. However, the zero crossing time obtained from the output of zero crossing detectors (ZRC) 134-140 made up of comparators is a continuous time value. Consequently, if zero crossing detectors (ZRC) 134-140 and phase difference detectors 142, 144 are made up of synchronized digital circuits as is, the measurement precision of the zero crossing time will depend on the clock frequency. Thus, there is a demand for an ultra-high-speed digital circuit that can operate at a clock frequency of tens of thousand of GHz in order to realize the same precision as that of the analog circuit, which is almost impossible to realize with the current digital technology. Because the zero crossing detectors (ZRC) 134-140 and phase difference detectors 142, 144 cannot be realized with synchronized digital circuits, even if only equalizers (EQ) 118-124 and low-pass filter (LPF) 148 are digitized, it is still difficult to connect the first- and second-stage circuits digitally. As a result, in the prior art, it is necessary that all of the functions of the first-stage analog front end part 106 be realized with analog circuits.
An object of the invention is to solve the aforementioned problems of the prior art by providing a tracking error detection method and circuit that enables high-precision detection of tracking errors with a digital circuit at a relatively low speed and with a small circuit scale.
Another object of the invention is to provide a tracking servo method and circuit that can perform high-precision tracking control with a digital circuit at a relatively low speed and with a small circuit scale.
Another object of the invention is to provide a method for high-precision detection of the phase difference of digital signals with a digital circuit at a relatively low speed and with a small circuit scale.